Picture transmission system



March 17, 1936. WEE 2,034,162

PICTURE TRANSMISSION SYSTEM Filed Ma 51, 1950 INVENTOR C.J. F. TW EED Patented Mar. 17, 1936 UNITED STATES PATENT OFFICE PICTURE TRANSMISSION SYSTEM Application May 31, 1930, Serial No. 457,723 In Great Britain June 28, 1929 1 Claim.

This invention relates to picture and like telegraph systems, and apparatus for use therein. More particularly, the invention relates to picture telegraph systems of the kind in which 5 the picture or other message is inscribed in conducting ink upo'n a message form from which it is picked up for transmission.

According to thisinvention, a message form for use in a picture or like telegraph system of the kind referred to is made of insulating material,

such as paper, capable of absorbing to some extent the conductive ink with which the message is to be inscribed, and said paper or other material is coated on one side with a conducting or semi-conducting material. In use the message is inscribed on the uncoated surface of the insulating material, and the ink soaks through said material forming a high or semi-high resistance path to the conducting layer at those places at which 20 inscription has been made. The resistance of the path to the conducting layer will, of course, vary with circumstances, but in practice said resistance will generally lie between 1000 ohms and 2 or 3 megohms.

In one form of construction, a message form consists of detail paper coated on one side with India ink or the material known as Aquadag. In another arrangement, metal coated paper, such as that known as Mansbridge paper, is employed. The paper employed should, of course, be sufiiciently absorbent to permit the ink with which the message is written to soak through to the conducting layer, but it should not be too absorbent otherwise a conducting coating of ink may go right through in places and give a blotchy effect.

The inscribed message form may be picked up and the message transmitted by means of a metal scanning stylus in the following manner, for example:

The form is wrapped round a metal cylinder with the conducting layer against the cylinder and scanned by a metal stylus connected through a resistance of, say, 10,000 ohms to the grid of a triode whose plate circuit is completed through a suitable output device and an anode battery to the cathode. The cathode is connected through a battery to the metal cylinder and also through a bias battery and a resistance of, say, 5 megohms to the grid. The various batteries and resistances are so proportioned that when the metal stylus is not in contact with an inked portion of the form i. e., where there is no marking, no plate current will flow in the valve. When, however, the stylus 55 comes in contact with a marked portion of the form, the battery connected between the valve cathode and the cylinder applies positive potential to the grid through the marked portion, and, in consequence, plate current will flow. In this arrangement the output will be intermittent unidirectional current, but obviously if a carrier frequency is required, an alternator or other oscillator may be conveniently interposed.

Another construction of message form which has been found very satisfactory in practice consists in very thin so-called Mansbridge paper which is mounted coated side down on ordinary good quality writing paper which serves to give stiifness to the writing surface. The mounting may be effected with ordinary paste or gum, care being taken to see that the thin tissue paper does not crinkle. One edge of the Mansbridge paper is left over-lapping and this is turned over so as to provide a coated margin through which contact can be made on to the cylinder upon which the message form is placed in order that it may be picked up by the scanning stylus. As regards the inks which may be employed, a solution of Aquadag of rather thick consistency so as to prevent its spreading on the somewhat absorbent paper has been found most suitable.

The accompanying drawing shows a suitable circuit arrangement for picking up a message from a message form in accordance with this invention. This circuit arrangement has the advantage that it operates between stops, i. e., it operates in such a manner as to be substantially independent of variation in the resistance and the writing upon the message form.

In the drawing, Fig. 1 shows schematically a transmitting circuit as embodied in this invention, while Fig. 2 illustrates one form of suitable message blank to be used in transmitting a message.

Referring to the drawing, I is the cylinder upon which the message form is mounted and which is caused by means not shown to move in such manner that the message form thereon is explored by a stylus 2. In series with the cylinder and stylus is a battery 3, a resistance 4 (for example, of 5 megohms) and a bias battery 5, said resistance 4 and battery 5 being connected as shown across the grid cathode space of a thermionic valve 6. An anode voltage of 100, for example, volts is applied to the anode of valve 6 through a resistance 1 of 50,000 ohms, for example, and the anode of said valve 6 is connected through two resistances 8, 9, of say 1 megohm each, to a source of negative potential, not shown, which serves as a source of grid bias potential for the following valves. The junction point of the two resistances 8 and 9 is connected to the middle point of the secondary winding of a transformer I0, said secondary winding being connected to the grids of two valves ll, l2, arranged in push-pull. An audio frequency is applied at the terminals l3 to the primary of the transformer l so that the valves ll, l2, are in push-pull as respects this audio frequency. The plates of the valves l I, I2, are connected together through a primary of the transformer M the mid point of said primary being taken to a source of anode potential of say 150 volts. The terminals 15 of the secondary of the transformer 14 serve as the output ter-' minals of the whole device.

With this arrangement, when the stylus is 7 moved over a portion of the message form upon which there is no writing, i. e., when there is an open circuit between the stylus 2 and the cylinder I the valve 6 is conductive, the said valve being made conductive by virtue of the bias potential applied from the battery 5 through the resistance 4. In these circumstances the valves ll, l2 will not bias any audio frequency since the constant negative bias will be of greater efiect than the positive potential applied to the plate of the valve 6. When contact is made through the writing between the stylus 2 and the cylinder l, the valve 6 is completely backed off and the rise of the anode tension voltage becomes available on its plate. This voltage is greater than the constant negative bias applied to the valves H, I2, and in consequence the grids of these valves become positive and the audio frequency note is therefore passed. The direct current feeds for the valves II and I2 are carefully balanced so that key clicks are not heard in the output circuit.

Having now described my invention, what I claim and desire to secure by Letters Patent is the following:

A message form for use in picture telegraph systems and the like which comprises a partially ink absorbent material, a conducting material backed upon one side of the absorbent material, a stiffening material to which said conductive backing is secured and so arranged with respect thereto as to provide an overlapping portion, said overlapping portion being folded back to form contact with the picture telegraph system drum or the like.

CYRIL JOHN FRANCIS TWEED. 

